Process for treating a cathodically chromated metal surface



"United States Patent 01 ice PROCESS FOR TREATING A CATHODICALLY CHROMATED METAL SURFACE Yoichi Kitamura, Yokohama, and Tsuneo Inui, Kudamatsu-shi, Japan, assignors to Toyo Kohan Co., Ltd.,

Tokyo, Japan, a corporation of Japan No Drawing. Filed Oct. 22, 1965, Ser. No. 502,277

Claims priority, application Japan, Nov. 12, 1964, 39/63,648 Int. Cl. C23f 7/26; C23b 11/00 US. Cl. 204-35 1 Claim ABSTRACT OF THE DISCLOSURE The present invention relates to a process for improving the properties of the coating formed in the cathodic chromating of metal surfaces.

Cathodic chromating is a process in which hexavalent chromium ions derived from chromic acid, chromates or dichromates in a treating solution are electrolytically reduced, mainly to trivalent state, to form a protective coating of a hydrated chromium oxide complex on the cathode metal. In the present specification the expression hydrated chromium oxide coating is employed to designate the film formed by chromating.

An analysis of the film formed shows that it contains no metals other than chromium, which is not present as simple metallic chromium. The analysis also shows that the film contains very little of the anion which was originally present in the chromating solution as an additron agent.

When the =wetted hydrated chromium oxide coating formed is immediately dried at an appropriate temperature after the chromating, the anion in the coating is fixed as a member of a co-ordinated complex structure of the coating. In a wetted state, however, the anion is not firmly fixed in the structure and behaves as if it was physically absorbed.

As shown in the specific examples, the conditions for a cathodic chromating process are quite different from those for chromium plating, dip chromating and anodic chromating. A coating formed in chromium plating is simple metallic chromium and thecoating formed by dip chromating is mainly a chromium chromate and in some cases is a mixture of a chromate and an oxide of the coated metal. The coating formed by anodic chromating is a mixture of a chromate and an oxide of the coated metal.

In the cathodic chromating process, various addition agents such as boric acid, phosphoric acid, sulfuric acid, disulfonic acid, nitric acid, acetic acid, or their respective salts and halogen compounds are added to the electrolyte, which mainly contains hexavalent chromium ion derived from chromic acid, chromates or dichromates as described in US. Pats. Nos. 2,733,199,, 2,780,592, 2,769,774, 2,998,- 316 and 3,032,487.

Of these additive agents; such strongly acidic and cor- 3,519,542 Patented July 7, 1970 rosive anions as sulfate ion, nitrate ion or chlorate ion are concerned in electrolytic reduction of hexavalent chromium ion, and are physically adsorbed in the hydrated chromium oxide film on metal surface. The adsorbed anion in the film is water soluble and is dissolved out of the film under highly humid conditions to accelerate corrosion of cathodically chromated metal.

Consequently, the additive agent which gives a strongly acidic anion is necessary to form an excellent film in cathodic chromating but it has a detrimental effect to the corrosion resistance of the formed film. Therefore, the adsorbed anion should be thoroughly removed or should be surely restrained.

Furthermore, the sensitivity of the hydrated chromium oxide film to oil adsorption has also a detrimental effect to the lacquer adhesion, especially, in the case of direct lacquering after degreasing a lubricant on the film, because the adsorbed oil on the fihn is very difficult to be removed by conventional degreasing and causes poor adhesion. Accordingly, the sensitivity of the film to oil adsorption should be properly improved within the limit not to impair the original lacquer adhesion property.

In order to restrain the strongly acidic anion adsorbed in the hydrated chromium oxide film and to-improve the adsorption property of the film, an immediate immersion of cathodically chromated metal in an aqueous solution containing at least one water-soluble or waterdispersible cationic surface active agent selected from the group consisting of aliphatic primary amine acetates, quaternary ammonium salts, pyridinium salts, picolinium salts, polyoxyethylene alkyl amines and polycondensation products obtained by the reaction of dicyandiamides and formaldehyde is most effective.

The following cationic surface active agents are used in the present invention.

(1) Aliphatic primary amine acetates.-Aliphatic primary amine acetates are the compounds having a formula of RNH COOCH wherein R is an aliphatic hydrocarbon group having 8-18 carbon atoms, and showing cationic activity in water.

For example, undecylamine acetate heptadecylamine acetate CH (CH NH COOCH and octadecylamine acetate CH (CH NH COOCH (2) Quaternary ammonium salts.Quaternary ammonium salts are the compounds having a formula of [RRRRN] X wherein R is an alkyl group having l-S carbon atoms, R is a member selected from the group consisting of saturated and unsaturated aliphatic hydrocarbon group having 1-22 carbon atoms, benzyl group and alkanol group having l-5 carbon atoms, R" is an aliphatic hydrocarbon group having 8-22 carbon atoms, X is halogen atom or hydroxyl group, and showing cationic activity in water, and also their derivatives having at least one ester bond, one ether bond or one amide bond in the molecules, and showing cationic activity in water.

For example, trimethyloctadecyl ammonium chloride dimethylcetylbenzyl ammonium chloride 0113mm) 1 1 orb-Q Cl dimethylheptadecyl-Z-hydroxyethyl ammonium chloride CH: crm cm) 1 CHzCHzOH] Cl CH2 diethylbenzyllauramidoethyl ammonium chloride C2115 CHZ(CH2)10CONHC2H4 I l oH2- or 32115 trimethyllauroyloxymethyl ammonium chloride [CH (CH COOCH N(CH ]Cl (3) Pyridinium salts.-Pyridinium salts are the compounds having a formula of wherein R is an aliphatic hydrocarbon group having 12-18 carbon atoms, X is halogen atom, and showing cationic activity in water, and also their derivatives having at least one ester bond, one ether bond or one amide bond in the molecules and showing cationic activity in water.

For example, cetylpyridinium bromide [cnucmmarQ] Br octadecyloxymethyl pyridinium chloride [crmcmmoom-rrQ] c1 lauramidomethyl pyridiniurn chloride (4) Picolinium salts.Picolinium salts are the compounds having a formula of stearamidomethyl picolinium chloride [CHa(CH2)mCONHCHaNQCHaICl cetyloxymethyl picolinium chloride random) 150 onmQormcr (5) Polyoxyethylene alkyl amines.Polyoxyethylene alkyl amines are the compounds having a formula of (CH2CH20)nH (CH2CH20)mH or RNH(CH CH O),,H wherein R is an aliphatic hydrocarbon group having 8-22 carbon atoms, n or m is a number of ethyleneoxide (n, m1=8-7-18) and, showing cationic activity in water.

For example, tetradecaoxyethylene pentadecylamine 3( 2)14 2' 2 )14 dioctaoxyethylene heptadecylamine (CHz-CHzOhH CH3(CH2)1|N (CH2-CHzO)sH (6) Polycondensation product obtained by the reaction of dicyanodiamides and formaldehyde, and showing cationic activity in water.-For example, a polycondensation product of dicyanodiamide [NH (NH=)CNH CN] and formaldehyde [HCHO].

"A polycondensation product of dicyanodiamidine and formaldehyde [HCHO].

A polycondensation product of guanidine and formaldehyde [HCHO].

A polycondensation product of diguanidine and formaldehyde [HCHO].

These cationic surface active agents can be used alone or by mixture.

The treatment according to the present invention is effective for restraining an adsorbed anion in hydrated chromium oxide film to prevent a cathodically chromated metal from rusting under highly humid conditions.

The treatment is also suitable for increasing the lacquer adhesion effect on an cathodically chromated metal in all respects, and for improving the lacquer adhesion property, especially in the case of lacquer coating on the mechanically shaped metal after degreasing a lubricating oil.

In the treatment, cationic surface active agents above mentioned can easily adsorb an anion adsorbed in the hydrated chromium oxide film in wetted state and make a desirable barrier for oil adsorption.

The conditions required for performing the process described in the present invention will be summarized as follows:

(1) Concentration of cationic surface active agent 1-50 g./liter (2) Temperature-30-70 C.

(3) Dipping time-1-6O seconds The optimum concentration of cationic surface active agent covers the range from 1 g./liter to 50 g./liter.

Generally, an increase of concentration results in an increase in an amount of cationic surface active agent to be adsorbed into the formed film. However, when the concentration of cationic surface active agent exceeds 50 g./ liter, the hydrated chromium oxide filrn becomes bluish yellow and shows no more improvement.

When the concentration is less than 1 g./liter, the process needs longer treating time which is not practical for the short time of commercial operation.

An increase of temperature results in an increasing effect on the improvement, but in actual operation, a higher temperature increases an evaporation loss of water and an decomposition of cationic surface active agent. The operation should be preferably controlled at a temperature below C.

On the other hand, a decrease temperature results in a decrease of an adsorbing power of cationic surface active agent into the formed film, and at a temperature below 30" C., the process requires an impracticably long treating time.

Therefore, it is preferable to conduct the operation in the temperature range of between 30 C. and 70 C.

An increase in dipping time results in an increase in an adsorbed amount of cationic surface active agent into the formed film, however, in the concentrations of treating solution of 1-50 g./liter and in the temperature range of 30-70 C., the dipping time of 1-60 seconds may be sufiiciently effective for the improvement of corrosion resistance and lacquer adhesion property of the film.

When the dipping treatment is conducted in a very short time less than 1 second, the process requires higher concentration and higher temperature than those described above and it is practically impossible in commercial operation.

When the film formed in cathodic chromating is forced to dry, a dipping treatment in an aqueous solution containing cationic surface active agent is less effective because the dried film behaves as a hard barrier against the action of the surface active agent. Therefore, the treatment should be immediately performed in wetted state immediately after the forming of film by cathodic chromating.

The iron and steel sheets treated according to the present invention can withstand outdoor exposure tests for two months in winter and for 15 days in summer and also resist salt spray tests for 48 hours without any sign of rust.

They further resist humidity chamber tests in a relative humidity of 90% at 40 C. for one week without any sign of discoloration of the film.

Such a high corrosion resistance of coating as in the present invention has not heretofore been attained by conventional treatment.

After applying an about micron film of modified alkyd or modified epoxy lacquer on steel sheet treated according to the present invention, the lacquered sheet was deeply drawn to make the cup with a drawing ratio of "2.2 and having a screw-shaped ridge on the cup wall to make a screw-shaped cup; when tested by adhesive tape, the lacquered side of the cup showed no peeling out.

In the case of lacquer coating on the mechanically shaped metal after degreasing a lubricating oil, superior lacquer adhesion was also found in the electrolytically chromated metal treated according to the present invention.

Other features of the lacquer adhesion property will be described in the following examples.

Metals treated according to the present invention showed good heat resistance up to 300 C., and weldability as good as the base metals.

As metals to be treated according to the present invention, the conventional metals to be treated in cathodic chromating under ordinary conditions, such as iron, steel, aluminum, chromium, nickel, zinc, tin, cadmium, magnesium, copper, titanium, their respective alloys and metals having the surface thereof are suitable.

The examples of the present invention are as follows:

EXAMPLE 1 A 0.25 mm. thick cold rolled low carbon steel sheet, the so-called black plate was cathodically cleaned for 20 seconds at a current density of 4 a./sq. dm. at 70 C. in a 7% sodium hydroxide solution, then rinsed with water, pickled for 10 seconds at room temperature in a 7% sulfuric acid, again rinsed with water and immediately treated cathodically using a lead antimony alloy anode under the following condition in the chromating solution of the following composition, immediately rinsed with water, and immediately dipped in the treating solution of the present invention under the condition given below, and again immediately rinsed with water and dried.

(1) The electrolyte and the condition of cathodic chromating:

Chromic acid-50 g./liter Sulfuric acid-0.2 g./ liter Hydrofluosilicic acid0.3 g./liter Temperature50 C. Electric current density-2O a./sq. dm. Time10 seconds 6 (2) The treating condition of the present invention:

Trimethyloctadecyl ammonium chloride50 g./ liter Dipping time-30 seconds A transparent, bluish purple film was formed and the steel sheet thus treated showed very little rusty spots after being subjected to a salt spray test by a 5% sodium chloride solution for 48 hours at 35 C. and it showed no rusty spot and no discoloration when subjected to a humidity chamber test with a relative humidity of for one week at 40 C.

EXAMPLE 2 A 0.5 mm. cold rolled low carbon steel sheet was subjected to the same pretreatment and the same cathodic chromating, as described in Example 1, and then immediately rinsed with water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water and dried.

A polycondensation product between formaldehyde and dicyanodiamide-IO g./ liter Temperature50 C.

Dipping time2 seconds The film formed was bluish yellow and transparent.

The steel sheet thus treated was applied with about 30 mg./sq. dm. of machine oil No. 120, degreased after 24 hours standing still with trichloroethylene vapor for 2 minutes, then coated with about 20 microns of melamine modified alkyd enamel and subjected to a cross cut test which showed no peeling of the enamel was found.

Furthermore, after the steel ball having a diameter of 12.7 mm. and a weight of 500 g. was dropped from a height of 20 cm. on the enamel coated steel sheet surface thus treated, the impact part is tested by adhesive tape, and no adhesion loss of enamel was also found.

EXAMPLE 3 The same kind of cold rolled low carbon steel sheet as in Example 1 was subjected to the same pretreatment as described in Example 1, and then immediately treated cathodically using a lead antimony alloy anode under the following condition in the chromating solution of the following composition, again immediately rinsed with water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water and dried.

(1) The electrolyte and the condition of cathodic chromating:

Chromic acid-60 g./ liter Phenol-2, 4-disulfonic acid-0.6 g./liter Temperature50 C. Current density-20 a./ sq. dm. Time20 seconds (2) The treating condition of the present invention:

Octadecylamine acetate-1 g./liter Temperature-55 C. Dipping time-60 seconds The film formed was bluish yellow and transparent and the steel sheet thus treated was found to show almost no rusty spots when subjected to the same salt spray test as in Example 1 for 48 hours and when the steel sheet treated was coated with about 20 microns of melamine modified alkyd enamel and subjected to the same salt spray test as in Example 1 for hours, no blister of the enamel was found.

EXAMPLE 4 The same kind of cold rolled low carbon steel sheet as described in Example 1 was subjected to the same pretreatment as described in Example 1 and then in mediately treated cathodically using a lead antimony alloy anode under the following condition in the chromating solution of the following composition, again immediately rinsed with water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water and dried.

(1) The electrolyte and the condition of cathodic chromating:

Sodium dichromate-SOO g./liter Chromium sulfate g./liter Ammonium sulfate g./liter Temperature50 C.

Current density a./sq. dm.

Time--8 seconds The treating condition of the present invention:

Cetyl pyridinium chloride-5 g./liter Temperature- C.

Dipping time20 seconds A transparent, bluish purple film was formed and it had the similar corrosion resistance to those in Example 1.

EXAMPLE 5 The same kind of cold rolled low carbon steel sheet as described in Example 1 was subjected to the same pretreatment and the same cathodic chromating, as described in Example 1, and then immediately rinsed with water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water and dried.

Dimethyloleylbenzyl ammonium chloride10 g./liter Di-octaoxyethylene stearylamine10 g./liter Temperature-70 C.

Dipping time5 seconds A transparent, yellowish and bluish purple film was formed. When the steel sheet thus treated was coated with about 10 microns of modified alkyd white enamel and deeply drawn to the cup with a drawing ratio of 2.2, no adhesion loss of the enamel was found on the cupwall. Furthermore, when the deeply drawn cup was immersed in a 3% boiling solution of sodium chloride for one hour, no adhesion loss was also found.

EXAMPLE 6 A 0.5 mm. aluminum sheet was cleaned for 10 seconds at room temperature in a 2% solution of sodium bicarbonate, rinsed with water and then immediately subjected to the same cathodic chromating as described in Example 1, then immediately rinsed with water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water and dried.

A polycondensation product between formaldehyde and dicyanodiamide5 g./ liter Temperature C.

Dipping time20 seconds The film formed was colorless and transparent, and when the aluminum sheet thus treated was subjected to a porosity test by cupric sulfate solution for two hours, no porosity was found.

EXAMPLE 7 A galvanized steel sheet with 0.15% aluminum was treated cathodically using a lead antimony alloy anode under the following condition in the chromating solution of the following composition and then immediately s rinsed with Water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water and dried.

(1) The electrolyte and the condition of cathodic chromating:

Sodium dichromate-100 g./liter Sulfuric acid2 g./liter Temperature-50 C.

Current density5 a./sq. dm.

Time1 second The treating condition of the present invention:

Trimethyloctadecyl ammonium chloride--30 g./liter Temperature50 C.

Dipping time-10 seconds The formed film was colorless and transparent and when the galvanized steel sheet thus treated was coated with about 20 microns of melamine modified alkyd enamel and subjected to a cross cut test and an impact test, no adhesion loss of the enamel was found.

What we claim is:

1. A process for improving the corrosion resistance and reducing oil adsorption in a wet hydrated chromium oxide coating cathodically deposited on a cathode metal selected from the group consisting of iron and steel which comprises rinsing the 'wet chromated metal with water prior to drying, restraining entrapped anions selected from the group consisting of sulfate, nitrate and chlorate anions by immediately immersing said metal in an aqueous solution at a temperature of 30 to C. for a period of 1 to 60 seconds containing from 1 to 50 grams per liter of at least one cationic surface active agent selected from the group consisting of polycondensation products of dicyanodiamide and formaldehyde, dicyanodiamidine and formaldehyde, guanidine ond formaldehyde, and diguanide and formaldehyde rinsing the treated. hydrated chromium oxide coating and drying the same.

References Cited UNITED STATES PATENTS 2,163,768 '6/1939 Tanner 1486.14 2,270,386 1/1942 Sloan 148-6.2 2,428,364 10/1947 -Frager 1486.14 2,918,390 l2/1959 Brown 148-62 3,136,663 6/1964 McDonald 1486.15 3,202,550 8/1965 Grossmann et al. 148-61 3,257,244 6/1966 Barkman 1486.2 3,261,710 7/1966 [Waldmann 117 -49 3,012,917 12/1961 Riou et al. 1486.14 2,837,432 6/1958 Drigot et al. 10614 3,210,184 10/1965 Uhlig 961 3,318,716 5/1967 Schuster et al. 10614 XR 3,337,431 8/ 1967 Kitamura et al. 204--56 3,288,555 11/1966 Napier et al. 106--14 XR 2,582,733 1/1952 Zimmer et al. 10614 FOREIGN PATENTS 986,455 3/ 1965 Great Britain. 600,021 6/ 1960 Canada.

JOHN H. MACK, Primary Examiner W. B. VAN SISE, Assistant Examiner US. Cl. X.R. 

